The use of silicon nitride ceramics in a number of high temperature structural applications has been proposed. The advantages of this material in such applications include its higher relative flexural strength and fracture toughness at elevated temperatures. Unfortunately, since silicon nitride is a mostly covalently bonded ceramic, it is difficult to densify fully in its pure state. Additives are necessary to promote a glassy grain boundary phase that aids in densification. It is the presence of the glassy silicate phase that limits the performance of silicon nitride at high temperatures. This glassy phase softens and melts with catastrophic effects on the mechanical properties of the ceramic.
One method for eliminating this glassy phase is described in U.S. Pat. No. 4,264,550, where a mixture of silicon nitride powder containing SiO.sub.2 as an oxide surface coating and Y.sub.2 O.sub.3 powder is heated to 1000.degree. to 1400.degree. C. under a pressure of at least 2000 psi to permit a nucleating reaction to take place. The mixture is then heated to a temperature of 1680.degree. to 1750.degree. C. under pressure. The resulting pressed body is claimed to contain fully crystallized grain boundary phases of Si.sub.3 N.sub.4.SiO.sub.2.Y.sub.2 O.sub.3.
Various silicon nitride compositions containing metal silicides have been disclosed. For example, U.S. Pat. No. 4,407,971, discloses a sintered ceramic body comprising yttrium oxide, aluminum oxide, aluminum nitride and 0.1 to 5% by weight of at least one silicide of Mg, Ca, Ti, V, Cr, Mn, Zr, Nb, Mo, Ta and W. U.S. Pat. No. 4,612,296 discloses a high toughness silicon nitride sintered body containing at least one silicide or carbide of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W. U.S. Pat. No. 4,983,554 discloses a sintered silicon nitride ceramic containing aluminum nitride, yttrium oxide, and molybdenum disilicide. U.S. Pat. No. 5,023,214 discloses a sintered Si.sub.3 N.sub.4 product containing a sintering aid and 1 to 80% of a silicide of Fe, Ni or Co. U.S. Pat. No. 5,094,986 (Matsumoto and Rosenthal), discloses a process for preparing a sintered silicon nitride ceramic containing 2% to 6% of a silicide of Fe, Ni or Co, at least 50% of which is a high metal content silicide. None of these references discloses a method for eliminating the glassy silicate phase in the silicon nitride ceramic by deoxygenating the sintered ceramic in a vacuum.